We are studying the reactions of osmium tetroxide - ligand systems with thymine and uracil derivatives in order to develop reagents capable of reaction with specific pairs and triplets of base residues in polynucleotides. The ultimate aim of this work is the determination of the base sequence in polynucleotides by direct visualization of single osmium atoms in the electron microscope. We are also interested in the reactions of both osmium (VI) and osmium (VIII) reagents with 5-RNA and its components so as to be able to synthesize reagents useful in the preparation of heavy atom isomorphous derivatives for x-ray crystallographic analysis. Our most general interest is in a systematic study of the reactions of osmium tetroxide and its reduction products with tissue components so as to provide electron microscopists, histologists, and pathologists with a firm chemical basis for the interpretation of their observations. BIBLIOGRAPHIC REFERENCES: Reactions of Oxo-Osmium Ligand Complexes with Isopentenyl Adenine and its Nucleoside. J. A. Ragazzo and E. J. Behrman, Bioinorganic Chem., in press (1976). On the Questions of Phosphate Complexes of Osmium Tetroxide: Kinetics of Reactions with Nucleotides and Dinucleoside Monophosphates. R. L. Clark and E. J. Behrman, Bioinorganic Chem., in press (1976).